Why use label in docker-compose.yml, can't environment do the job?

Question

I am learning docker now. I am trying to figure out what kind of problem Docker label can solve.

I can understand why use label in Dockerfile, e.g add build-related metadata, but I still don't get why using it in docker-compose.yml? What is the difference between using labels vs environment? I assume there will be different use cases but I just can't figure it out.

Can someone give me some practical example?

Thanks

Answer 1

docker-compose.yml is used by docker-compose utility to build and run the services which you have defined in docker-compose.yml
While working with docker-compose we can use two thing

1. docker-compose build this will build the services which is defined under docker-compose.yml but in order to run this services it has to have a image which is with docker-engine if you do docker image ls you find the images which is built up with the docker-compose and inspect it there you find a label which defines the metadata of that particular image.
2. docker-compose up this will run the services which is built up in docker-container build now this running container has to have some metadata like env this is set with enviroment in docker-compose.yml

P.S. :- This is my first answer in stack overflow. If you didn't get just give a comment I will try to explain my best.

Answer 2

Another reason to use labels in docker-compose is to flag your containers as part of this docker-compose suite of containers, as opposed to other purposes each docker image might get used for.

Here's an example docker-compose.yml that shares labels across two services:

x-common-labels: &common-labels
  my.project.environment: "my project"
  my.project.maintainer: "[email protected]"

services:
  s1:
    image: somebodyelse/someimage
    labels:
      <<: *common-labels
    # ...
  s2:
    build:
      context: .
    image: my/s2
    labels:
    <<: *common-labels
    # ...

Then you can do things like this to just kill this project's containers.

docker rm -f $(docker container ls --format "{{.ID}}" --filter "label=my.project.environment")

re: labels vs. environment variables

Labels are only available to the docker and docker-compose commands on your host.

Environment variables are also available at run-time inside the docker container.